Electroless deposition of metal is effected by immersing catalyzed substrates into metal solutions, e.g. of soluble nickel, cobalt or copper, a reducing agent and a chelant. Although substrates can be catalyzed with a variety of metals from Group 1B or Group 8, palladium, despite its cost, is often the catalyst of choice due to its activity. Maintaining adhesion of the catalyst to the surface is of considerable importance since loosely adhered catalytic metal can be washed from the surface in the agitation of the plating bath causing depletion of the metal value of the plating bath as uncontrolled metal deposition occurs, resulting in what is commonly referred to as a "crashed" bath.
Adhesion of catalytic metal can be promoted by incorporating the metal in a polymeric film. For instance, Morgan et al. in U.S. Pat. No. 4,910,072 discloses selective catalytic activation of catalytically inert polymeric films comprising complexes of a polymer and a compound of a Group 1B or 8 metal where the films are prepared from preferably anhydrous solutions.
Vaughn in U.S. Pat. No. 5,082,734 discloses the use of aqueous coating systems to selectively provide catalytic polymeric surfaces. In many cases the polymeric coating is sufficiently resistant to the solubilizing components of aqueous plating solutions, e.g. acid, base or formaldehyde, to allow plating of a adequately adhesive metal layer. In other cases the catalytic activity is so high that an occluding layer of metal is deposited before the aqueous plating bath can adversely affect the catalytic layer. However, in certain other cases, e.g. where the polymer contains hydrophilic groups and/or low levels of catalytic metal, immersion in an aqueous plating bath promotes a variety of undesirable effects, e.g. dissolution of catalytic metal, swelling or solubilization of the polymer coating with loss of catalytic metal, weakened adhesion between the substrate and catalytic polymer layers or weakened adhesion between the metal and catalytic polymer layers.
Prior art practitioners have disclosed the use of a variety of crosslinking agents to minimize any tendency for hydroxy-containing polymers such as polyvinyl alcohol (PVOH) and cellulosic polymers to solubilize and be washed away. For instance, Vaughn in U.S. Pat. No. 5,082,734 discloses the use of terephthaldicarboxaldehyde, glutaraldehyde or dialdehyde starch as crosslinking agents. Lenoble in U.S. Pat. No. 3,615,471 discloses electroless deposition on catalytic polymeric photoresistant coatings from aqueous solutions of palladium dichloride, PVOH and a light activated crosslinker such as potassium bichromate. Yudelson et al. in U.S. Pat. No. 3,719,490 disclose electroless deposition on catalytic coatings from solutions of actinic light activated palladium, a polymeric binder such as gelatin or PVOH with a boric acid crosslinker. Such crosslinking agents provide marginal improvement against the adverse effects of electroless plating baths.